1. Field of the Invention
The present invention relates to a device and method for reproducing recorded information from a magneto-optical recording medium.
2. Discussion of Related Art
A magneto-optical recording medium employed in recording information has been practically utilized in situations where high-density rewritable media has been desired. The magneto-optical recording medium using a recording layer made of an amorphous alloy of a rare-earth element and a transition metal has been especially useful.
A method for recording information is described as follows. A laser beam is collimated onto a face of an optical recording medium to increase temperature of a small spot of the recording layer to about 150-200C. When the temperature exceeds a curie temperature (Tc) a magnetization phenomenon on the spot of the optical recording medium disappears. Then a biasing magnetic field is applied to orient the magnetic field of the spot, and the spot is allowed to cool to indoor temperature. The magnetic field of the spots is observable as recording marks (or pits).
A process for recording the information on a magneto-optical recording medium will be hereinafter illustrated referring to FIGS. 1 and 2. FIG. 1 is a block diagram showing a general type of recording device and FIG. 2 represents timing diagrams of the operation of the general recording device shown in FIG. 1.
First, a channel clock generator 9 generates a channel clock signal 10 based on information pre-formatted on an optical disk. In response to the channel clock signal 10, a laser driver 11 controls pulse beam emissions of a laser diode 1. Such pulsed laser beam 2 is irradiated as an optics spot 4 onto an optical disk 8 through an object lens 3.
Meanwhile, a data signal generator 6 forms a modulated magnetic field 7 by using a magnetic head 5 adjacent to the optical disk 8.
As shown in FIGS. 2A to 2E, the optical spot 4 is pulsed onto a face of the optical disk 8 synchronous with the frequency of the channel clock signal 10. In such a process, the pulsing of laser beam 21, the modulating of magnetic field 7, and irradiating of the optic spot 4 occur synchronously to the channel clock signal 10 to record information onto the optical disk 8. Marks are piled up on the optical disk 8 and recorded by such irradiated optic spots 4. Magnetic pits having a shorter mark length than the optic spot 4 are recorded by such recording method, and such method is a well-known technique exhibited in Japanese Opening Patent No. Pyeong 1-292603.
In one method for reproducing the information written by the above recording method scheme on the optical disk, it is known in the art that the laser beam having a constant output is condensed and irradiated on the surface of the optical recording medium. The condensed optic spot is reflected on the surface of the magneto-optical recording medium, and a polarization state of the laser beam is changed by a Kerr effect. By detecting polarization state change of the reflected light, the information written on the disk can be read.
As shown in FIG. 3A, however, a problem occurs when density is increased. To increase density, the length of the magnetic mark becomes shorter. As a result, the optic spot becomes larger in relation to the length of the magnetic mark. Thus reproducing information becomes more error prone as the density increases.
A super resolution technique has been recently tried to overcome such shortcoming. A magnetically induced super resolution (MSR) using an exchange-combination force among multilayer films is provided as one settlement method out of the recent techniques.
One scheme for using an inner plane magnetization film in such MSR technique is offered in FIG. 4. In this scheme, the magneto-optical recording medium is made up of two layer films of an exchange-combination structure formed by a reproducing layer having comparatively a little coercive force and a recording layer having comparatively a strong coercive force.
The reproducing layer serves as the inner plane magnetization film at indoor temperature, meantime over a constant temperature, its magnetization direction is changed and the reproducing layer represents a perpendicular magnetization.
The recording layer is formed by the perpendicular magnetization film to keep the information. When the optical beam is irradiated onto the reproducing layer to read the information, the innerplane magnetization is changed to the perpendicular magnetization, on the reproducing layer, by a pole Kerr effect, at a high temperature region of the optical spot, namely at the central region where the temperature is over a threshold value as shown in FIG. 4. That is, the high temperature region of the reproducing layer is changed to the same direction as a magnetic field direction of the recording layer.
While, at low temperature regions peripheral to the high temperature region of the optical spot, the magnetization of the recording layer is masked since the pole Kerr phenomenon does not occur. Accordingly, a reproducing operation of the super resolution is available by properly selecting power of a reproducing laser beam, since recorded information is reproduced only at the high temperature region corresponding to the central portion of the optical spot.
One example of a device for reproducing the record information from the optical recording medium is shown in FIG. 5, and the timing diagrams are shown in FIGS. 6A-6E. This reproducing device employs a pulsed laser beam as the reproducing light. A reproducing clock generator 58 outputs a reproducing clock signal shown in FIG. 6A and a pulse generator 57 outputs a pulse type signal based on the reproducing clock signal. A laser driver 56 drives laser diode 55 in response to the pulsed signal. The pulsed laser beam emitted as shown in FIG. 6B from the laser diode 55 is condensed onto an optical recording medium 51 through a condenser 54 and an object lens 52. An optical spot condensed on the optical recording medium 51 is reflected and enters a first polarized beam splitter 53 through the object lens 52. The optical spot from the first polarized beam splitter 53 enters a second polarized beam splitter 59, and at this time, a P polarization element is let through and an S polarization element is reflected.
The P polarization element and the S polarization element are respectively condensed by a first photo detector 61 and a second photo detector 60, and converted into electrical signals. The converted signals are inputted to a differential amplifier 62 to be differentially amplified, and applied to a reproducing signal processor 63. The reproducing signal processor 63 processes the signal from the differential amplifier 62 and outputs a bit signal, namely a binary signal as detected information. FIG. 6D represents writing marks written on the optical recording medium 51 shown in FIG. 5. In FIG. 6D, hatched marks indicate a binary signal of a high level and white marks indicates a binary signal of a low level.
A magnetic amplifying magneto-optical system (MAMMOS) has been used as a technique to overcome problems related to high density writing as depicted in FIG. 3. Such technique realizes higher writing density by employing a magnetic film of two layers formed by a recording layer and an enlarged reproducing layer.
In this technique, an alternating reproducing magnetic field is applied onto the recording medium, to thereby enlarge a minute magnetic domain of a high density disk, copy the domain on the enlarged reproducing layer, and thus increase a detected reproducing signal. As shown in FIG. 6C, the alternating magnetic field is made up of a magnetic field signal of an up magnetization direction corresponding to a binary signal of a high level and a magnetic field signal of a down magnetization direction corresponding to a binary signal of a low level.
The alternating reproducing magnetic field is applied onto the recording medium regardless of the recording marks shown in FIG. 6D. That is, the magnetic field direction of the recording mark is not checked. Thus the field direction of the mark and the field direction of the alternating field may oppose each other. In such a situation, the reproduced signal may not be detected definitely if the magnetization direction of the applied alternating magnetic field signal is opposite to the signal of the recording mark.
Referring to FIGS. 6C to 6E, if the magnetization direction of the applied alternating magnetic field signal is the same as the signal of the recording mark, a level of the detected reproducing signal appears as L1 or L3. If they are not same as each other, the level of the reproducing signal shows as L2, as shown in FIG. 6E. The levels L1 and L3 are read as respective binary signals of a high level and a low level, but the level L2 can not be read as high or low. Reproducing signals of level L2 thus causes errors. For example, as shown in FIG. 6E, the detected level at positions P.sub.1 and P.sub.2 is L.sub.2. However, the actual levels should be L.sub.1 and L.sub.3 for positions P.sub.1 and P.sub.2, respectively.
When using the MAMMOS technique, however, errors are unavoidable, since the alternating magnetic field is applied without regard to the magnetic field direction of the mark. Finally, even if a zero crossing or a level slicing is executed by the levels for detecting the high binary signal and the low binary signal, errors still may occur. The written binary information can not be read definitely in general in the method for level slicing the detected reproducing signal.